Thermal treatment of aluminum base alloy articles



United States Patent 3,253,965 THERMAL TREATMENT OF ALUMINUM BASE ALLOYARTICLES Charles B. Criner, Southbury, Conn., assignor to AlumiuumCompany of America, Pittsburgh, Pa., :1 c0rporation of Pennsylvania NoDrawing. Filed Sept. 11, 1963, Ser. No. 308,085 7 Claims. (Cl. 148-11.5)

This application is a continuation-in-part of my application Serial No.98,295, filed March 27, 1961, now abandoned.

This invention relates to improving the tensile strength and resistanceto corrosion of articles of certain thermally treated aluminum basealloys and it is particularly concerned with the interposition of a coldworking step before the final thermal treatment of the articles.

It is Well known that aluminum base alloys containing certain elements,which are soluble in solid aluminum, respond to the thermal treatment,known as solution heat treatment, which brings about a solution of atleast a portion of undissolved elements or intermetallic compounds. Whensuch alloys are rapidly cooled, i.e. quenched, from the solution heattreating temperature, a metastable condition is created. If the alloysin that condition are subjected to a further thermal treatment, byheating to a temperature somewhat above room temperature, at least apart of the dissolved elements or compounds is precipitated with aresultant increase in strength and hardness as compared to the untreatedalloy or even the alloy which has only received a solution heattreatment.

It has also been recognized that aluminum base alloys containingmagnesium in combination with other ele ments, which have a substantialsolubility in solid aluminum, can be cold worked following the quenchingand before the precipitation treatment with a resultant in crease in thetensile and yield strengths, as compared to the alloys which have notreceived such a cold working. In referring to cold work it is to beunderstood that this refers to any of the operations such as rolling,pressing, drawing, and the like which effect a reduction in crosssectional thickness of the alloy article where those operations areconducted at room temperature or close to that temperature and thearticle becomes work hardened. The term cold work also embracesoperations which create work hardening strains with little or noreduction in cross sectional thickness. For example, flattening orstretching of a warped article introduces work hardening strains. Alloysof the foregoing type respond relatively rapidly to the precipitationtreatment, either with or without any intermediate cold working, andconsequently have presented no difiiculty in attaining a high strength.In such alloys that have been artificially aged the cold working doesnot improve the resistance to corrosion.

Aluminum base alloys composed essentially of aluminum and 4 to 7% byweight of copper and free from magnesium do not follow the pattern ofthose alloys referred to hereinabove in the response to theprecipitation treatment and the increase in strength attained by thattreatment where cold working precedes precipitation. Furthermore, thisdifference in response adversely affects the resistance to corrosion inthe absence of cold work. As a consequence, alloys of this type haveenjoyed but limited use although they possess advantageous properties inother respects than strength at room temperature and resistance tocorrosion.

My invention is directed .to improving the strength and resistance tocorrosion of articles of precipitation hardened aluminum-copper alloysof the foregoing type Patented May 31, 1956 ice and has as its primaryobject the provision of a method for treating articles of such alloys.It is also an object of this invention to provide a method of treating awrought article or a portion thereof composed of an essentially binaryaluminum-copper alloy whereby a higher strength and a better resistanceto corrosion is developed than in the same alloy which has not receivedany cold work before the precipitation hardening treatment.

I have discovered that the strength, particularly the yield strength, ofarticles of essentially binary alloys of aluminum and 4 to 7% copper andsubstantially free from magnesium and zinc can be considerably improvedand the resistance to corrosion simultaneously increased by interposinga cold working step between solution and the precipitation treatments,the amount of cold work being closely related to the temperature andlength of the precipitation treatment. By means of this treatment it hasbeen possible to develop a tensile strength on the order of 69,000p.s.i. and a yield strength of 53,000 p.s.i.

with an elongation of 11% as compared to a tensile strength of 60,000p.s.i., a yield strength of 43,000 p.s.i. and an elongation of 11% whenthe same alloy is treated in the conventional manner without anyintermediate cold work. It has also been found that the resistance tocorrosion of the alloy article is improved as compared with the samecomposition which has been given a precipitation treatment to developthe maximum strength and hardness but without any intervening cold work.

The type of alloy which is improved by my process, as mentioned above,is one that is composed essentially of aluminum and from 4 to 7% by'Weight of copper and free from magnesium and zinc except as they occuras impurities. The alloy should contain at least 4% copper in order toattain a high strength while on the other hand more than 7% introducesproblems in working the alloy and the increased copper content does notproduce a significant increase in strength. when the alloy is coldWorked and precipitation hardened. Other elements which aresubstantially insoluble in solid aluminum may be present in relativelysmall amounts. For example, at least one of the group of hardeningelements consisting of manganese, titanium, vanadium, zirconium,molybdenum, tungsten, chromium, boron, nickel, cobalt, tantalum andniobium can be present in the following amounts: 0.15 to 1.2% manganese,0.05 to 0.20% vanadium, 0.05 .to 0.30% zirconium, and 0.1 to 0.25% eachof titanium, molybdenum, tungsten, chromium, boron, nickel, cobalt,tantalum and niobium. The total amount of these elements with. theexception of manganese, vanadium and zirconium, should not exceed about0.25%. If the machinability of the alloy is to be improved at least oneof the group of low melting point elements consisting of lead andbismuth may be added in amounts of 0.1 to 0.75% each, the total notexceeding 1.5%. Neither of the foregoing elements, as far as is known,adversely affect the physical properties of the alloy, and beingsubstantially insoluble in solid aluminum they do not interfere with thesolution and precipitation of the copper.- By the same token the alloysare substantially free from elements such as magnesium and zinc whichare soluble in solid aluminum, except as they may occur as impurities.The magnesium impurity content should not exceed 0.02% and the zinccontent should not be over 0.25%

The usual iron and silicon impurities can be tolerated but it isadvisable to restrict the iron to a maximum of 0.5% and the siliconcontent should not exceed 0.3%.

The first step in my process consists of a solution heat treatment whichconsists of heating the alloy to a temperature between 900 and 1050 F.and holding within this range for a period of time, on the order of A to12 hours, to effect substantially complete solution of the copper. Atthe end of the holding period the alloy is to be quickly cooled to muchlower temperature, generally room temperature or one not far removedfrom room temperature. The chilling may be accomplished in any one ofseveral conventional ways as by quenching in water, or by a water sprayor even an air blast, if the article is not too thick.

Upon attaining room temperature or close to it, the alloy article isstrain hardened either by a reduction in cross section by rolling,pressing, drawing or by other known metal working methods or simply byflattening or stretching the article, for example, the work which isdone in straightening a warped product. To gain a substantial increasein strength it is necessary to effect a reduction in cross section andgenerally this should be on the order of at least 1% to gain the desiredimprovement. More than 20% does not offer any advantage. The amount ofcold work that can be employed depends upon the nature of the articleand the ease of making reductions, thus, only a relatively smallreduction can be made on extrusions and tubing by stretching, but sheetcan be reduced by a much larger amount. Generally from 1 to cold work ispreferred since most of the advantages are obtained within this rangeand because in some cases, the size of the product precludes greateramounts of cold work.

The relationship is more precisely expressed in equations related to theyield strength and maximum resistance to corrosion. The equationrelating to the yield strength is as follows:

5 ing the cold worked product to a temperature between 300 and 400 F.for a period of 1 to 48 hours, the choice of particular conditions beingdetermined from the equations stated above. At temperatures below 300 F.the

length of time required to gain the desired values becomes too long forpractical purposes while above 400 F. the opposite condition prevails,the period of exposure is so short that control becomes difficult.

The alloys to which the above-described treatment is applied should bein the wrought condition, as distinguished from castings. The wroughtforms may be produced by any of the conventional methods such as byrolling, forging, extrusion, pressing, and the like. In many instancesthese are performed at elevated temperatures and hence are referred toas hot working operations as distinguished from those performed at roomtemperature.

Although the description of the process thus far has involved thetreatment of an entire article, it has been found that portions of anarticle may be treated with benefit by the process. For example, inwelding members of this alloywith filler metal of a similar composition,or

even where only the filler metal is of this type of alloy, the weldedstructure is solution heat treated, quenched, the welded area coldworked and finally precipitation hardened. In this manner the weld beadand worked area adjoining it are improved.

where 1 t=time required to reach maximum yield strength T =precipitationhardening temperature C W=percent cold work 51 B (0.01T) [1+0.01(CW)0.25]

The calculated length of time represents the minimum that should be usedin practice. Exceeding this period by two hours, for example, has but aslight effect on the yield strength, generally less than 3,000 psi. Theresistance to corrosion of a product which has been so treated isadequate for many applications.

To insure a maximum resistance to corrosion, with some sacrifice inyield strength, the minimum period of precipitation hardening should bedetermined in accordance with the following equation: 55

where t=time required to reach solution potential of 800 millivoltsT=precipitation hardening temperature CW: percent cold work The maximumresistance to corrosion is here considered to be attained when thesolution potential of the alloy has a value of not less than 800millivolts as determined in a standard aqueous solution of 3 /2 NaClcontaining 0.3% by volume of H 0 as measured against a standard calomelhalf cell.

- r In respect to the relationship of cold work to precipita- Myinvention is illustrated in the following examples.

Example 1 An alloy nominally composed of aluminum, 6.5% copper, 0.25%manganese, 0.1% vanadium, 0.15% zirconium 40 and 0.05% titanium wasmelted, cast into ingot form, hot

rolled to plate thickness, annealed and cold rolled to sheet 0.064 inchin thickness. Samples taken from the sheet were given a solution heattreatment at 1000 F. for /2 hour and quenched in cold water. One portionof the samples (A) was heated for 36 hours at 375 F. to produceprecipitation hardening. A second portion (B) was cold rolled with areduction in thickness of 1.5% and then heated to 350 F. for 18 hourswhich exceeds the minimum time determined from Equation 2. A thirdportion (C) was cold rolled with a reduction of 10% and precipitationhardened by heating to 325 F. for 14 hours which is close to the timecalculated from Equation 2. The average tensile properties of each ofthese groups is given in Table I below.

TABLE I.TENSILE PROPERTIES OF AlCu-MnVZr-Ti ALLOY The improvement inyield strength produced by the cold Work is especially notable.

Samples of the sheets were also exposed to standardized corrosion testswherein some of the samples were placed under a stress equivalent to 75%of the yield strength while other samples were not subjected to anystress. Both stressed and unstressed samples were alternately immersedand raised from an aqueous solution of 3.5% NaCl over a period of 12weeks. At the conclusion of the 12 week period the samples weresubjected to a tensile test and the loss in strength as compared to thatof samples of the original material was noted. The percentage loss instrength of the specimens is given in Table II below.

TABLE II.-PERCENT LOSS IN TENSILE STRENGTH FROM CORROSION UnstressedStressed of an alloy consisting essentially of aluminum and 4 to 7% byweight of copper, and free from magnesium and zinc except as impurities,said method comprising heating said articles to between 900 and 1050 F.for a period It is to be seen that the cold work not only increased thestrength of the alloy but that the resistance to corrosion was improvedappreciably which is unique among solution heat treated andprecipitation hardened aluminum base alloys.

Example 2 The benefit of cold working another type aluminumcopper alloyis illustrated in the case of an alloy nominally composed of aluminum,5.5% copper, 0.5% lead and 0.5% bismuth. An ingot of this alloy was hotrolled to rod form 2 inches in diameter. Sections were cut from the rod,solution heat treated at 975 F. for 2 /2 hours and quenched in coldwater. One group (D) was heated to 320 F. and held for 14 hours whilethe second group (B) was cold drawn with a reduction in cross section of20% before beingheated to 320 F. and held at that temperature for 14hours which is close to the time determined according to Equation 2. Theaverage tensile properties of the samples taken in a longitudinaldirection are given in Table III below.

TABLE IIL-TENSILE PROPERTIES OF Al-Cu-Pb-Bi ALLOYS Tensile Yield PercentGroup Strength, Strength, Elongap.s.i. p.s.i. tion Specimens from therespective bars were exposed to same alternate immersion test describedabove in both the stressed and unstressed conditions. The losses intensile strength are given in Table IV.

TABLE IV.PERCENT LOSS IN TENSILE STRENGTH FROM CORROSION UnstressedStressed and resistance to corrosion of wrought articles composed 75where t=time required to reach maximum yield strength T=precipitationhardening temperature C W=percent cold work.

2. The method according to claim 1 wherein the alloy also contains atleast one of the group of hardening elements composed of 0.15 to 1.5%manganese, 0.05 to 0.20% vanadium, 0.05 to 0.3% zirconium, and 0.01 to0.25% of titanium molybdenum, tungsten, chromium, boron, nickel, cobalt,tantalum and niobium, the total amount of said elements except formanganese, vanadium and zirconium not exceeding 0.25

3. The method according to claim 1 wherein the alloy also contains atleast one of the low melting point elements of the group composed oflead and bismuth in amounts of 0.1 to 0.75% each, the total notexceeding 1.5%.

4. The method according to claim 1 wherein the cold working is withinthe range of l to 10%.

5. The method of improving both the yield strength and resistance tocorrosion of welded articles in the welded area wherein the filler metalis composed of an alloy consisting essentially of aluminum and 4 to 7%by weight of copper, and free from magnesium and zinc except asimpurities, said method comprising heating said filler metal in saidwelded area to a temperature between 900 and 1050 F. or a period of A1to 12 hours, quenching said heated filler metal, cold working the fillermetal and the area immediately adjacent thereto from 1 to 20% andthereafter heating said cold worked metal to a temperature between 300and 400 F. for a period of 1 to 48 hours to induce precipitationhardening, and for a minimum length of time needed to develop a maximumyield strength, said time being determined from the equation:

where t=time required to reach maximum yield strength T=precipitationhardening temperature CW=percent cold work 51 B=Ant11g (0.O1T)[1+0.01(CW)- 6. The method of improving the resistance to corrosion andthe yield strength of wrought articles composed of an aluminum basealloy consisting essentially of aluminum and 4 to 7% by weight ofcopper, and free from magnesium and zinc except as impurities, saidmethod comprising heating said articles to between 900 and 1050 F. for aperiod of A to 12 hours, quenching said articles, cold working saidquenched articles from 1 to 20% and thereafter heating said cold workedarticles to between 300 and 400 F. for a period of 1 to 48 hours toinduce precipitation hardening, and for a minimum length of timerequired to attain a solution potential of 800 millivolts, said timebeing determined from the equation:

where t=time required to reach solution potential of 800 millivoltsT=precipitation hardening temperature CW=percent cold Work.

7. The method according to claim 6 wherein the cold working is withinthe range of 1 to 10% References Cited by the Examiner UNITED STATESPATENTS 2,706,680 4/1955 Criner 75139 FOREIGN PATENTS 456,721 5/ 1949Canada. 458,636 8/1949 Canada. 443,909 3/ 1936 Great Britain. 738,07010/ 1955 Great Britain.

OTHER REFERENCES Physical Metallurgy of Aluminum Alloys, ASM, 1949, pp'.37, 204, 205

DAVID L, RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

H. F. SAITO, Assistant Examiner.

1. THE METHOD OF IMPROVING BOTH THE YIELD STRENGTH AND RESISTANCE TOCORROSION OF WROUGHT ARTICLES COMPOSED OF AN ALLOY CONSISTINGESSENTIALLY OF ALUMINUM AND 4 TO 7% BY WEIGHT OF COPPER, AND FREE FROMMAGNESIUM AND ZINC EXCEPT AS IMPURITIES, SAID METHOD COMPRISING HEATINGSAID ARTICLES TO BETWEEN 900 AND 1050*F. FOR A PERIOD OF 1/4 TO 12HOURS, QUENCHING SAID ARTICLES, COLD WORKING SAID QUENCHED ARTICLES FROM1 TO 20% AND THEREAFTER HEATING SAID COLD WORKED ARTICLES TO ATEMPERATURE BETWEEN 300 AND 400*F. FOR A PERIOD OF 1 TO 48 HOURS TOINDUCE PRECIPITATION HARDENING, AND FOR A MINIMUM LENGTH OF TIME NEEDEDTO DEVELOP A MAXIMUM YIELD STRENGTH, SAID TIME BEING DETERMINED FROM THEEQUATION: COMPLEX FRACTION WHERE